1. Field of the Invention
This invention is to be used in key telephone systems and other such push button systems. It is used to control the signals associated with the various lines going to and coming from a key telephone.
2. Description of the Prior Art
In the past line circuits have been designed around relays because they provide the isolation required of the telephone lines. In particular the incoming T (tip) and R (ring) require a large degree of isolation from other circuitry. Without this high degree of isolation, noise is introduced into the telephone lines.
It is common practice in the telephone industry to ring a station with a large 20 cycle AC voltage of approximately 100V R.M.S. To detect this signal a relay coil is utilized which provides isolation and immunity to large voltages such as the 100V of ringing. Since the coil floats with the line, it introduces no noise.
To detect the line current when a party goes off hook, a relay coil is also used. The line current operates the relay when DC current flows through the coil. Here, again, the relay coil floats with the line providing isolation to the line.
For terminating the line when the station party goes on hold, isolated relay contacts close to insert a resistor between T and R. The relays are actuated by logic signals and drivers. The line isolation of the T and R is achieved by the isolated relay contacts. Freedom from large line transients is also achieved.
In addition to the above, the same relays control audible and visual signals for local ringers and lamps. The contacts of these relays control large AC signals such as 100V AC for the ringer. These signals are isolated from other circuitry by the use of isolated contacts. The motor of the interrupter is also controlled by one set of contacts.
The logic and drivers for the relays utilize various logic schemes such as additional relays, transistors, diodes, and integrated circuits. The contacts of the relays are utilized extensively to form the logic by having the power or signals go from one relay to another.